and please create a child theme as the guide says to , because it will make customizing bbPress alot easier. you can edit all the bbPress templates that you have copied to your child theme safely if you go to appearance>editor and choose your child theme and find the bbPress templates/css, you can also use FTP and a advanced text editor like notepad++ to edit files on your cpu.

btw. I see many people work on files through the editor in WordPress, but this only allows edits of existing files, and only core files.
I think there is a need for a small tutorial on editors, root folders and ftp access for starters, I remember being very intimidated by my root folders at first.